Your quilt is pretty and very colorful. Why not have the final border be a piano key border with all the colors from the quilt?
While you are doing these calculations, don't forget that you will get some shrinkage when you wash and dry the quilt. I always allow at least 5 to 8 inches extra. I do not prewash my fabrics, so if you have prewashed, you may get less shrinkage.
As for the quilting, I am a user of panto's because my artist skills have not arrived. I'll leave the advice to those who are much better than I am.